<p>Microalgal oil has been increasingly studied as a feedstock for biodiesel production through transesterification reactions using heterogeneous catalysts. This route offers several benefits, including catalyst reuse, ease of separation, and improved safety, while addressing environmental and technical issues associated with using homogeneous acids and bases. Most studies use methanol for the transesterification, and few studies have investigated the transesterification of microalgal oil using ethanol. Beyond the environmental benefits of microalgae compared to plant-based biomass, replacing methanol with bioethanol is advantageous due to its lower cost and reduced toxicity. If the emulsion issue between the produced biodiesel and ethanol is resolved, ethanol could be a more environmentally friendly alternative for green fuel production. This study evaluated various metal oxides as catalysts for the transesterification of rapeseed oil using ethanol as both reagent and solvent to improve miscibility. From catalyst screening, CaO showed the highest fatty acid ethyl esters yield and this catalyst was then tested at different reaction times in two systems (round-bottom flask and autoclave reactor) for the transesterification of both rapeseed and microalgal (<i>Scenedesmus</i> sp.) oil. The highest reaction yield was 86.0% for rapeseed oil and 81.3% for microalgal oil using 114:1 ethanol: oil molar ratio with CaO in an autoclave reactor. This work addresses the limited studies on ethanol in microalgal oil transesterification, demonstrating the effectiveness of CaO as a catalyst. It highlights the potential of ethanol as a greener, cost-effective alternative to methanol for biodiesel production.</p> Graphical Abstract <p></p>